1. Field of the Invention
The invention relates generally to aerogel composite materials and methods of making them. In particular, the invention relates to an aerogel structure having an electrically connected network of ruthenium dioxide deposited throughout the structure and to methods of making the composite.
2. Background of the Related Art
Ruthenium dioxide (RuO.sub.2), one of the platinum group metal oxides, is an important industrial material due to its metallic electrical conductivity (RuO.sub.2 single crystal conductivity approaches 10.sup.5 S/cm at 25.degree. C.) along with its excellent chemical and thermal stability and diffusion barrier properties. These characteristics have led to the use of ruthenium dioxide in electrodes for catalysis, electrolysis, photovoltaic devices, capacitors, thick and thin film resistors, etc.
Many techniques based on chemical vapor deposition (CVD) have been developed for depositing dense RuO.sub.2 films on flat substrates, including: sputtering or evaporating ruthenium metal in the presence of oxygen; plasma decomposition of Ru-bearing gases by glow discharge; thermal or photolytic decomposition of one of several organometallic precursors. Deposition by reacting oxygen with evaporated metal vapor can be activated by applying a dc current or r.f. radiation, as described in U.S. Pat. No. 5,055,319 to Bunshah et al. In Yuan et al. "Low-Temperature Chemical Vapor Deposition of Ruthenium Dioxide from Ruthenium Tetroxide: A Simple Approach to High-Purity RuO2 Films" Chem. Mater. 5 (1993) pp 908-910, incorporated herein by reference, the deposition of RuO.sub.4, which spontaneously reduces to RuO.sub.2, by CVD is described. The precursor was either RuO.sub.4 in a solution of water, pentane or carbon tetrachloride or pure RuO.sub.4 solid. Using this approach, RuO.sub.2 films 1-micron thick with resistivities of about 10.sup.-2 ohm-cm were prepared.
For many RuO.sub.2 applications such as catalytic and sensing applications, it is desirable that the RuO.sub.2 material have the highest possible surface area in order to maximize the number of reaction sites. Conventionally, porous RuO.sub.2 electrodes are prepared by dip-coating a substrate in RuCl.sub.3 solution and heating in air to decompose the salt to RuO.sub.2. A technique for increasing the porosity of RuO.sub.2 by doping the ruthenium chloride solution with lanthanum chloride and, after firing, removing the lanthanum oxide by dissolving in sulfuric acid is described in Takasu et al., J. Alloys Comp. 261 (1997) p. 172, incorporated herein by reference. The RuO.sub.2 is stable and is five times "rougher" than the sample prepared without La doping. These materials have good electrical conductivity, but the surface area is still fairly low.
Aerogels are a class of materials typified by extremely high surface area (up to 1000 m.sup.2 /g) and porosity (up to greater than 99%). These properties are generally achieved by extracting the solvent from the pores of a wet porous gel under supercritical conditions, thereby avoiding shrinkage caused by capillary forces that develop during ambient drying. Although a wide range of aerogel compositions are possible, silica is the most widely studied. When formed by catalyzed hydration and polycondensation of a metal alkoxide solution, followed by exchange of pore-filling solvent with, and then removal of, supercritical carbon dioxide, silica forms a relatively robust monolith with extremely low electrical and thermal conductivity.
Efforts have been made previously to develop techniques to deposit Ru oxide on porous substrates. U.S. Pat.No. 4,298,439 to Gafney, incorporated herein by reference, claims a process for adsorbing RuCl.sub.3 in aqueous solution in/on a porous glass and then oxidizing in air at 120.degree. C. for one week to obtain the oxide. There is no indication whether this process resulted in a conductive film. Miller et al, J. Electrochem Soc. 144 (1997) L309, incorporated herein by reference, discloses a method of depositing Ru oxide by heating a volatile organometallic Ru compound in the presence of carbon aerogel in a sealed reactor. Decomposing the deposited organometallic by heating in flowing argon resulted in 2-nm Ru particles dispersed throughout the aerogel pores. The Ru/carbon aerogel composite had significantly higher specific capacitance than the untreated aerogel, but the Ru phase did not form its own electrically conductive network.